The present invention relates to a method of drying a formed cellular body to improve the surface integrity and handling of such body during subsequent processing steps.
It is becoming increasingly common to form highly loaded plastically deformable mixtures into complex structures such as a honeycomb configuration by extrusion. Immediately upon forming, the resulting structure is generally soft and fragile, and easily damaged or distorted, making the handling and processing of such structures very difficult. To overcome this problem, commercial extrusions of green bodies such as tubes, rods, and cellular structures are accomplished by using extremely stiff batches. The stiff batch is necessary for the extruded material to reflect the geometry of the extrusion die and for the extruded batch to maintain its shape during further processing. If the batch is too soft then the extruded body will sag or distort. However, stiff batch will not flow without significantly large applied pressure, typically in excess of 900 psi (6,000 KPa), depending on the desired extrudate geometry. The use of high pressures requires specially designed extrusion hardware which can withstand the high pressure and maintain close tolerances in the final ware. Standard extrusion dies require batches which must be entirely self supporting throughout the drying process.
Extrusion dies are often made from hardened steels or steels with wear resistant coatings. Without such coatings, a stiff viscous batch tends to wear away die material, and leads to a variation in the extruded product over time. These dies are difficult and expensive to manufacture and must be replaced frequently due to wear.
One method of producing extruded bodies is to use hot extrusion of thermal-plastic batch in which the viscosity of the batch is lowered by heating the extruder and die assembly. This allows for easier extrusion at lower pressures but requires thermal-plastic binder materials which constitute a large volume fraction of the batch, for example, as much as 30 weight percent. In contrast, the binder in conventional extrusion typically constitutes no more than 10 weight percent of the batch, and more typically can be 2 to 5 weight percent and as low as 1 weight percent of the batch. In both cases the binder or thermal-plastic is typically removed from the green body by heating in the process of forming the final product. Because of the high weight percent of thermal-plastic binder, the cost of thermal-plastic extrusion is higher and any removal of the plastic or binder is more difficult and more expensive.
As the complexity and size (frontal area) of the extrudate increases, it becomes increasingly difficult to maintain uniform extrusion pressure and therefore, flow uniformity of the stiff batches used in commercial processes. The high extrusion pressures which are required tend to exert a large bending stress on the center of the die. As a result, large frontal area dies tend to bow, yielding inconsistent products. Also, as the frontal area increases, the total force required increases, requiring larger and more expensive equipment. At an extrusion pressure of 1000 psi (6,890 KPa), 18,000 pounds (80 KN) of force are required to extrude a 3 inch by 6 inch (7.62 cm.times.15.24 cm) body, while 288,000 pounds (1,280 KN) are required to extrude a 1 foot by 2 foot (30.5 cm.times.71 cm) body. A softer batch would reduce the pressure required for extrusion of large frontal area bodies. However, such softer batches also tend to sag during the drying process.
Various methods have been proposed for processing green bodies from raw material slurries. For example, a method has been suggested for processing slender or thin sheets or fibers using very liquid slurries which contain a solvent by contacting the sheet or fiber with a solidifying liquid to dissolve and remove the solvent from the body.
Recently, it has been suggested to contact a porous body formed of a high solids matrix containing thermally gelable organic binder, with a rapid-setting compound for a duration and at a temperature sufficient to stiffen the body, and removing any excess compound from the body, the rapid-setting compound being characterized by (a) having a strong water affinity to pull water out of the matrix, and (b) having appropriate molecular blocking configuration and/or larger stearic hindrance to prevent the water from re-entering the matrix. It has also been suggested to contact such extrudates with electromagnetic radiation within certain frequencies either while the bodies are in the die, or as the extrudates exit the die.
There have been no suggestions to extrude complex shaped articles such as honeycombs from solvent-containing, soft batches by contacting the articles with a drying liquid in which the solvent and optionally, portions of the solid particles are soluble.